1. Field of the Invention
This invention relates generally to the field of computer buses and more specifically to controllable active terminators for computer buses.
2. Description of Related Art
Computer systems typically use an electronic bus to communicate signals between various computing devices such as a processor, a memory, and input/output (I/O) devices. A computer bus typically to communicates address, data, and control signals between the computing devices connected thereto. In a digital computer system, the signals transferred over the bus are typically high frequency signals ranging between ground and V.sub.cc. The signals are typically driven on the bus by bus drivers incorporated into each of the computing devices connected thereto.
A common computer peripheral interface is the Small Computer Systems Interface (SCSI). The SCSI bus is becoming increasingly popular because it reduces the I/O bottle neck in existing computer systems. However, as the frequency of the signals transmitted over the SCSI bus increase, and as the distances between the computer peripherals connected to the bus also increase, transmission line effects associated with the SCSI bus degrade the integrity of the signals transmitted over the bus. To provide optimum signal power transfer between computing devices and to reduce signal reflection due to impedance mismatches on the SCSI bus, the bus must be terminated in such a way that the impedance of the terminator is close to the impedance of the bus and the terminator should supply the maximum allowable current to the bus.
The SCSI-2 computer bus requires operation at 10 megabytes per second over a bus cable length of less than of 6 meters. Termination has become a critical design factor due to the increases in signalling frequencies, the potential distances between SCSI stubs, differences in cable design and other factors. For example, due to impedance mismatches on the SCSI bus, the ack and reg control signals may be reflected causing the ack and reg lines to be double clocked. One source of signal reflection is due to mismatches between cables having slightly different impedances. Another source of reflection is due to the "stubs" (i.e., the length of cable that is coupled to the primary SCSI bus) and the position of the stubs on the SCSI bus. Most efforts in improving the performance of the SCSI bus have been directed at reducing the deleterious effects of the reflection problems associated with the SCSI bus. Various termination techniques have been attempted.
For example, passive terminators have been used for terminating single-ended SCSI-1 devices. FIG. 1 shows a typical passive terminator which provided reliable operation even when the SCSI-1 bus was fully configured and run at maximum cable lengths. As shown in FIG. 1, the passive terminator 100 terminates a bus signal line 102 into a resistive load consisting of a 220 ohm resistor 104 connected to the terminator power line 106 and a 330 ohm resistor 108 connected to ground. The effective resistance of the passive terminator is equal to 132 ohms. Disadvantageously, the passive terminator 100 provided a resistive path between the terminator power line 106 and ground even when the signal line 102 is not active (i.e., at high impedance). Therefore, the passive terminator 100 used in the prior art computer bus systems dissipates power continuously, even when all of the bus signal lines 102 are negated. For a terminator with a nominal power supply voltage of 5.0 volts, the passive terminator 100 dissipates 50 mW (10.00 milliamps.times.5.0 volts) for every inactive bus signal line 102. Another disadvantage of the passive terminator 100 shown in FIG. 1 is that the Thevenin voltage is not regulated and thus varies with variations in the terminator power supply 106. For example, a terminator power variation between 4.25 volts and 5.25 volts causes the output voltage to vary from 2.55 volts to 3.15 volts. Consequently, a correspondingly large variation in the current supplied to an asserted bus signal line (e.g., signal line 102) through the 220 ohm resistor 104 is produced. High tolerance resistors were required in order to limit the output current provided on bus signal line 102. The costs of manufacturing the passive terminator 100 were thereby increased.
Generally, the passive terminators of the prior art are modular devices which must be manually inserted onto the computer bus to provide termination and manually removed from the bus to remove termination when, for example, the computer bus is to be extended. Disadvantageously, physical access to the computer bus (e.g., opening enclosures which protect the device drivers and the bus) is required to effect such bus termination modifications. Therefore, a need exists for a controllable active terminator for a computer bus which can be seIectively switchably connected to and disconnected from the computer bus.
Active bus terminators, such as the Boulay terminator shown in FIG. 2, have also been developed. Active termination of the computer bus provides a potential reduction of reflection problems caused by impedance mismatches on the bus. In general, the prior art active terminators attempt to reduce the reflection by compensating for voltage drops and maintaining a constant stable voltage to the terminating equipment resistors. The Boulay terminator 200 shown in FIG. 2 uses an active voltage regulation technique to improve noise immunity and reduce average power dissipation. The linear voltage regulator 202 produces a voltage source of 2.85 volts on line 204. As shown in FIG. 2, the 2.85 volts is provided in series with a plurality of termination resistors 206 which are connected to a plurality of computer bus signal lines 208. Typically, the plurality of termination resistors 206 comprise 110 ohm resistors having a 1% tolerance. The scheme shown in FIG. 2 is suited to terminate bus lines having a relatively low characteristic impedance, which is fairly common. Because the computer bus signal lines 208 are terminated by an active voltage regulation scheme, noise immunity is improved and a substantial reduction and average power dissipation is reduced. The reduction and average power dissipation is achieved because a negated or high impedance line conducts no current through its respective termination resistor 206. Thus, the only power dissipated by the Boulay terminator 200 for the negated line is the power dissipated by the linear voltage regulator 202. Typically, the linear voltage regulator 202 dissipates between 5 to 10 milliamps of current.
Furthermore, because the Thevenin voltage is regulated, the output current is substantially immune to variations in termination power. Disadvantageously, in order to provide the maximum current on the computer bus signal lines 208 and meet the impedance specification of the SCSI standard, the termination resistors 206 must have relatively low tolerance values. When the resistors are included on an integrated circuit device together with the regulator 202, laser trimming is required to produce resistors 206 having these low tolerance values. Consequently, the manufacturing costs associated with the prior art Boulay terminators were increased.
One attempt at providing a controllable bus terminator having voltage regulation is disclosed in the prior art and shown in FIG. 3. The voltage regulator 106 includes a differential amplifier 120, a transistor 140, a switchable current source 150, and a power transistor 142. The emitter of the power transistor 142 is coupled to the collectors of the output transistors 100 as shown. The differential amplifier 120 works with the transistor 140 and the switchable current source 150 in order to control the conduction of the power transistor 142 and thereby hold the regulator's output voltage 127 constant. Disadvantageously, the voltage regulator 106 supplies current directly to the output transistors 100 via the collectors of the transistors 100. The current capacity of the output of the terminator is consequently limited by the current capacity of the regulator 106. Further, the voltage regulator 106 components are necessarily large due to the design of the terminator. For example, the power transistor 142 comprises a relatively large transistor which is necessary to source current to the plurality of output transistors 100.
Furthermore, the voltage regulator 106 uses a transistor 130 to provide feedback from the output transistors 100 to the differential amplifier 120. As a result, the feedback provided the voltage regulator 106 is somewhat unpredictable due to voltage variations produced by the transistor 130 arising from variations in the output current. The design shown in FIG. 3 uses multiple output switches or transistors 100 to connect and disconnect the active terminator onto the computer bus. The use of multiple output switches disadvantageously increases the power dissipation and size of the terminator.
Moreover, the output transistors 100 operate in saturation when supplying current to the output signal lines 172. As a result, the controllable bus terminator shown in FIG. 3 is a relatively high current device. The output transistors 100 also operate in saturation during shutdown.
Therefore, there is a need for a controllable bus terminator having output transistors which operate in their linear range and is thus a relatively low current terminating device. There is also a need for a controllable bus terminator which can take advantage of field effect transistor (FET) technology to produce a lower power and relatively small active bus termination device. The present invention provides such a device.